I was listening to the radio in the car this morning and I heard an ad for the local phone company about why is was better to stick with their service than to switch to getting your phone service through the internet. One issue with internet phone services is 911 isn’t available during a power failure.
And that got me thinking.
Telephone lines provide enough power to run your phone. And they’re routed differently than normal power lines so a disaster that takes out the power may leave the phone lines unscathed.
So … in an emergency could you draw enough power from your phone line to do anything useful? I’m imagining a little LED emergency light that you could plug into a phone jack. Or a little emergency radio. Do such products exist already and I just don’t know about them? Or is using the phone line this way illegal or prohibited?
Phone lines do carry a moderate amount of power; enough, certainly, to run a little electromagnetic ringer. However, I’m pretty sure that drawing this power off for uses other than the phone would be considered abuse of the service.
The 90 volt AC ring signal isn’t the power that Pochacco had in mind. The -48 volts that’s on the line when your phone is on-hook is delivered through a very high DC loop resistance in the neighborhood of 10 megohms, so available current is minuscule - about 5 microamps.
Off-hook “talking power” is in the area of 35 milliamps, but is AC, and at about 6-8 volts.
It is possible to get a useful amount of power when the line is off-hook, but that would also mean you’d be unable to receive calls as your line would appear busy.
As humorous as that page is, a lot of those devices are technically possible, due to the fact that they actually run off of a battery. The power from the phone line is only used to charge the battery, so for most of the devices it’s only a matter of how long do you have to wait before the battery is charged up enough for use.
Of course, you can charge the batteries a lot faster from AC, but why let practical details like this stand in your way?
As for the OP, you can get enough power to light a handful of LEDs from the phone line, but not enough to make an emergency lighting system out of them. You could make a really low power radio receiver, but you would have to hold the earpiece up to your ear. You wouldn’t be able to make it much louder than the output of a telephone designed for hearing impaired people.